Types of Friendship

Best friend (or close friend): a person(s) with whom someone shares extremely strong interpersonal ties with as a friend.
Some examples are as follows:

Acquaintance: a friend, but sharing of emotional ties isn't present. An example would be a coworker with whom you enjoy eating lunch, but would not look to for emotional support.

Soulmate: the name given to someone who is considered the ultimate, true, and eternal half of the other's soul, in which the two are now and forever meant to be together.

Pen pal: people who have a relationship via postal correspondence. They may or may not have met each other in person and may share either love, friendship, or simply an acquaintance between each other.

Internet friendship: a form of friendship or romance which takes place over the Internet.

Comrade: means "ally", "friend", or "colleague" in a military or (usually) left-wing political connotation. This is the feeling of affinity that draws people together in time of war or when people have a mutual enemy or even a common goal. Friendship can be mistaken for comradeship. Former New York Times war correspondent Chris Hedges wrote:
“ We feel in wartime comradeship. We confuse this with friendship, with love. There are those, who will insist that the comradeship of war is love — the exotic glow that makes us in war feel as one people, one entity, is real, but this is part of war's intoxication. [...] Friends are predetermined; friendship takes place between men and women who possess an intellectual and emotional affinity for each other. But comradeship – that ecstatic bliss that comes with belonging to the crowd in wartime – is within our reach. We can all have comrades.[9] ”

As a war ends, or a common enemy recedes, many comrades return to being strangers, who lack friendship and have little in common.

Casual relationship or "Friends with benefits": the sexual or near-sexual and emotional relationship between two people who don't expect or demand to share a formal romantic relationship.

Boston marriage: an American term used in the nineteenth and twentieth centuries to denote two women that lived together in the same household independent of male support. Relationships were not necessarily sexual. It was used to quell fears of lesbians after World War I.

Blood brother or blood sister: may refer to people related by birth, or a circle of friends who swear loyalty by mingling the blood of each member together.

Cross-sex friendship is one that is defined by a person having a friend of the opposite sex: a male who has a female friend, or a female who has a male friend. Historically cross-sex friendships have been rare. This is caused by the fact that often men would labor in order to support themselves and their family, while women stayed at home and took care of the housework and children. The lack of contact led to men forming friendships exclusively with their colleagues, and women forming friendships with other stay at home mothers. However, as women attended schools more and as their presence in the workplace increased, the segregated friendship dynamic was altered, and cross-sex friendships began to increase.

Open relationship: a relationship, usually between two people, that agree each partner is free to have sexual intercourse with others outside the relationship. When this agreement is made between a married couple, it's called an open marriage.

Roommate: a person who shares a room or apartment (flat) with another person and do not share a familial or romantic relationship.

Imaginary friend: a non-physical friend created by a child. It may be seen as bad behavior or even taboo (some religious parents even consider their child to be possessed by an evil spirit), but is most commonly regarded as harmless, typical childhood behavior. The friend may or may not be human, and commonly serves a protective purpose.

Spiritual friendship: the buddhist ideal of kalyana-mitra, that is a relationship between friends with a common interest, though one person may have more knowledge and experience than the other. The relationship is the responsibility of both friends and both bring something to it.